My male cali was brought out of brumation early, he was a finiky eater last spring and summer (breeding) and never really got into "eating" once breeding season was over. He was losing weight too fast, and I was worried we would lose him. I tried mice and rats, and finally gave him a bucket full of pinkies and fuzzies, and lo and behold, he crawled in, killed most of them, and ate half of them. Now I feed him a bucket of fuzzies every few weeks, and he's putting on weight. HOpefully his brumation was long enough, as I have plans to breed him in a month or so. Meanwhile I'll be fattening him up.
IF that picture is a recent one, I would suggest that force feeding is an unwise decision. He looks healthy, and if his denying food has been over the winter, he has obviously planned on brumation, and nature has dictated this. You mentioend how much weight he lost.... from what weight to what weight? The actual weights will help many on here know if he has had a drastic reduction in weight, or a minor redcuction in weight. If you've kept him warm and tried to feed him while he has wanted to brumate, my guess is that he has spent more energy with the warmer temps, but had no appetite, so he's burned more calories.
Anyway, my two cents.
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~Sasheena